Organic electroluminescent elements that use an organic material as a luminescent material act to recombine holes injected from their anodes and electrons injected from their cathodes and to form excited molecules (excitons), which radiate energy to emit light when returning to a ground state.
In 1987, C. W. Tang et al. of the Eastman Kodak Company announced an organic electroluminescent element having organic films laminated between its anode and cathode, and thus realized emission of high luminance light at a low driving voltage (C. W. Tang et al., Applied Physics Letters, 1987, Vol. 51, p. 913: Nonpatent document 1).
Since the announcement by Tang et al., eager studies have been conducted of organic electroluminescent elements, that is, of their luminescence in three RGB primary colors, luminance improvement, stability, laminate structure and manufacturing method. The organic electroluminescent elements have already been in some practical use as a component of a display for the mobile telephone or car audio, and are regarded as promising as a component of a next-generation flat display which is an alternative to the liquid crystal display.
Many organic electroluminescent elements use an electron-transporting material in combination with a luminescent material.
The electron-transporting material is used for efficient transportation of electrons injected from the cathode to a luminescent layer and also for blocking of holes. Usable electron-transporting materials include an oxadiazole derivative and Alq3 (tris(8-hydroxyquinoline)aluminum), which is widely used as a green-color luminescent material.
Methods for forming an organic film for an organic electroluminescent element are roughly classified into two categories: dry methods and wet methods. The dry methods include a vacuum deposition method and a CVD method, while the wet methods include a spin coating method and an inkjet method.
The dry methods have an advantage of facilitating formation of a multi-layered film due to vacuum deposition. When used for the manufacture of organic electroluminescent elements, the dry methods facilitate formation of a multi-layered film comprising a hole-injecting layer, an electron-injecting layer, a hole-blocking layer and the like, to achieve an injection balance between holes and electrons. Thus, organic electroluminescent elements manufactured by the dry methods realize high efficiency and luminance, and display devices using such organic electroluminescent elements have already been in practical use. The thy methods, however, have a disadvantage of requiring large-scale apparatuses for the manufacture of elements having a large area, which lowers the productivity.
On the other hand, the wet methods have an advantage of permitting formation of a coating film having a large area, all at a time, which facilitates the manufacture of elements having a large area and increases the productivity. In terms of productivity and costs, therefore, the wet methods are superior to the dry methods. Especially for polymer materials, with which it is often difficult to form a thin film by vacuum deposition, the wet methods are mainly employed.
Japanese Unexamined Patent Publication No. 2002-63989 (Patent document 1) discloses a luminescent element containing an organic fluophor represented by the formula:R3P═Owherein R, the same or different from each other, are substituents such as an aryl group, at least one of R being a fluorescent skeleton.
Also, Japanese Unexamined Patent Publication No. 2004-204140 (Patent document 2) discloses a material for luminescent element represented by the formula:(Ar)3P═Owherein Ar, the same or different from each other, are an aryl group or a heteroaryl group, at least one of Ar being a naphthyl group linked at its α position, at least one of Ar containing either a fluorescent skeleton or a charge transport skeleton.
The above publication recites condensed rings such as phenanthryl, anthranyl, pyrenyl and perylenyl, as examples of fluorescent skeletons.
The structure of the phosphorus-containing organic compound of the present invention, however, is different from, though similar to, the structures of the organic fluophor and the material for luminescent element disclosed in the above publications, since it does not have a naphthyl group linked at its α position, a fluorescent skeleton or the like.
Patent document 1: Japanese Unexamined Patent Publication No. 2002-63989
Patent document 2: Japanese Unexamined Patent Publication No. 2004-204140
Nonpatent document 1: C. W. Tang et al., Applied Physics Letters, 1987,